1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier or a laser beam printer, which employs an electrophotographic printing method or an electrostatic recording method and includes a developing apparatus for visualizing a latent image formed on an image bearing member by adhering a developer thereto.
2. Related Background Art
Heretofore, in an image forming apparatus such as a copier using an electrophotographic printing method or an electrostatic recording method, an electrostatic latent image formed on an image bearing member such as a photosensitive drum is visualized by adhering a developer thereonto. Among developing apparatuses for use in such development, an example of one using a two-component developer containing a toner and a carrier is shown in FIG. 11.
In this example, in a developing apparatus 1, a first developing sleeve 8 as a first developer carrying member is disposed so as to be rotatable in an arrow direction shown in FIG. 11 in a developer container 2 for housing a two-component developer that contains a non-magnetic toner and a magnetic carrier as a developer T. Further, a developer regulating blade 9 for regulating the magnetic brush of the developer carried on the first developing sleeve 8 is disposed.
Further, a second developing sleeve 11 as a second developer carrying member is disposed so as to be rotatable in the same direction as a rotation direction of the first developing sleeve 8 in a region under the first developing sleeve 8, which is opposed to the first developing sleeve 8.
In addition, the developer container 2 is vertically partitioned by a partition wall 7 into an upper developing chamber 3 and a lower agitating chamber 4, in which developer feed screws 5 and 6 are disposed, respectively. The developer T circulates in the developing chamber 3 and the agitating chamber 4 by the above-described screws 5 and 6.
Note that the first developing sleeve 8 is a cylindrical member made of a non-magnetic material, in the inside of which a magnet roller 8a as first magnetic field generating means is provided in a non-rotating state. The magnet roller 8a includes a developing pole S2 and magnetic poles S1, N1, N2, and N3 which feed the developer. Among them, the first magnetic pole N3 and the second magnetic pole N1 which have the same polarity are adjacent to each other to form a repulsive magnetic field therebetween, and a barrier is formed against the developer.
Further, a second developing sleeve 11 is made of a non-magnetic material, in the inside of which a magnet roller 11a as second magnetic field generating means is provided in a non-rotating state in a similar way to the first developing sleeve 8. The magnet roller 11a includes three magnetic poles S3, S4, and N4. Among them, the third magnetic pole S3 and the fourth magnetic pole S4 which have the same polarity are adjacent to each other to form a repulsive magnetic field therebetween, and a barrier is formed against the developer.
The S2 pole of the magnet roller 8a and the N4 pole of the magnet roller 11a are the developing poles. The developer which stands like the ears by the S2 pole and the N4 pole develops a latent image formed on a photosensitive drum 10 by applying a developing bias as a superimposed voltage of a DC voltage and an AC voltage to the first developing sleeve 8 and the second developing sleeve 11.
In such two-component development as described above, at the time of developing a white ground portion, the toner in the developer at a developing nip portion is thrust in a direction toward the first developing sleeve 8 and the second developing sleeve 11 by an electric field formed of a voltage on-the photosensitive drum 10 and the developing bias.
Specifically, the toner is developed on the first developing sleeve 8 and the second developing sleeve 11 after developing the white ground portion. In other words, the toner is attached onto the first developing sleeve. Sand the second developing sleeve 11 and the surfaces of the first developing sleeve 8 and the second developing sleeve 11 turn into a contaminated state by the toner.
When a high-density portion is developed in the state where the first developing sleeve 8 and the second developing sleeve 11 are in the contaminated state by the toner, the toner that has contaminated the first developing sleeve 8 and the second developing sleeve 11 is developed on the photosensitive drum 10. Accordingly, there has sometimes occurred a malfunction that a density is increased only in the contaminated portion with regard to the electrostatic latent image with a uniform electric field on the photosensitive drum 10, that is, a phenomenon (so-called ghost) of dragging an image of the immediately previous rotation of the first developing sleeve 8 and the second developing sleeve 11.
Further, it is conceivable that the developer attached onto the first developing sleeve 8 and the second developing sleeve 11 for a long period of time rubs against other developers and members and is fused onto the first developing sleeve 8 and the second developing sleeve 11 owing to frictional heat.
When the toner is fused onto the surfaces of the first developing sleeve 8 and the second developing sleeve 11, a feed amount of the developer to the developing regions of the first developing sleeve 8 and the second developing sleeve 11 is lowered to thus lower an image density, and high-resistance layers made of fused matter on the surfaces of the first developing sleeve 8 and the second developing sleeve 11 are formed. Accordingly, even if the developing bias is applied to the developing region between the first developing sleeve 8 and the photosensitive drum 10 and the developing region between the second developing sleeve 11 and the photosensitive drum 10 at the time of the development, desired electric fields are not formed. As a result, a sufficient development effect brought by the developing bias has not been obtained. Therefore, sometimes the density decreases, and an image failure such as blank areas sometimes occurs.
The toner contamination mentioned above is more prone to occur in the second developing sleeve 11 than in the first developing sleeve 8. The reason is as follows. As for the developer on the first developing sleeve 8, an amount thereof passing the developer regulating blade 9 is regulated by the developer regulating blade 9, and the developer tends to accumulate between the S1 pole and N1 pole of the magnet roller 8a. Therefore, the toner contamination attached onto the first developing sleeve 8 is scraped off easily by being rubbed with the accumulated developer. On the other hand, the second developing sleeve 11 which does not include the developer regulating blade 9 does not have the accumulated developer, and accordingly, becomes prone to be contaminated by the toner more significantly.
As measures against the problems as described above, it is necessary to always keep the surface of the second developing sleeve 11 in a state where there is no toner contamination on the developing sleeve 11 by cleaning the toner adhering onto the second developing sleeve 11. With regard to this, the following proposals have been made heretofore.
There have been proposed: a construction, in which a Mylar tape, an elastic blade, or a fur brush is attached as a member for scraping off a toner adhering onto a developing sleeve to scrape off the toner on the developing sleeve in a contact manner; and a construction, in which a conductive roller is provided in a non-contact manner with respect to a developing sleeve, a voltage is applied between the conductive roller and the developing sleeve, and a toner adhering onto the developing sleeve is made to fly toward the conductive roller side to scrape off the toner on the developing sleeve (for example, refer to JP 10-312110 A and JP 2003-173086 A).
However, in the above-described constructions, the following problem occurs.
Specifically, the member for scraping off the toner adhering onto the developing sleeve is newly provided, and accordingly, requirements for space saving and cost reduction of the image forming apparatus using the electrophotographic printing method in recent years cannot be satisfied. It has not been preferable to provide such a scraping member in terms of design.